1. Field of the Invention
The present invention relates generally to apparatus and processes for producing particulate sulfur from molten sulfur and more particularly to apparatus and processes for producing relatively large and dense sulfur particles having a relatively low water content.
2. Discussion of the Background:
Sulfur, which is widely distributed in nature, is often considered, along with salt, coal and limestone, as being one of the four basic raw materials of the chemical industry. Important uses for sulfur include: acids and chemicals, agriculture, petroleum refining, ammonium sulfate, pulp and paper, textiles and rubber. Sulfur is produced commercially either as naturally occurring, "native," sulfur or as "recovered" sulfur which is a byproduct of many industrial processes, including the desulfurization of natural gas, crude petroleum, tar sands, oil shale, coal and geothermal fluids. Presently about as much recovered sulfur as native sulfur is produced.
One of the most important end uses of sulfur is for fertilizers and soil conditioners in agriculture, about half of all the sulfur produced being used for this purpose. In this regard, sulfur is essential to life, and all sulfur requirements for human and animal nutrition come from foods. The ultimate source of sulfur in food is soil from which sulfur is absorbed by plants as sulfate ions. Within plants, the sulfate ions are transformed into complex organic sulfur compounds which then enter the food chain. As an illustration of the importance of sulfur, protein, which is an essential constituent of all living cells and is required for building new tissue and replacing old tissue, is synthesized from such sulfurcontaining amino acids as methinone and cystine; other essential sulfur-containing substances are hormones and enzymes, as well as vitamin B.sub.1.
In addition to correcting sulfur deficiencies affecting plant growth, sulfur and/or its compounds are applied directly to soil for soil conditioning purposes such as correcting soil alkalinity, reacting with soil so that the soil releases other nutrient elements needed by plants, and acting as a soil ameliorant. Sulfur, usually in the form of organic matter and sulfates, also improves soil structure, increases its water-holding capacity, modifies soil reaction, and stimulates the growth of soil microorganisms. Indirectly, sulfuric acid made from sulfur is used to treat phosphate rock in the production of phosphate fertilizer materials for plant use.
Sulfuric acid also has a great many other uses, such as in oil refining and the making of dyes, pigments, textiles and plastics. Because sulfuric acid is so widely used and is one of the most versatile inorganic chemical known, about 80 to 85 percent of all sulfur consumed is first converted to (or produced as) sulfuric acid.
Largely because it is usually produced in liquid form, about ninety percent of all sulfur transported within the continental United States is shipped in the molten state. However, before sulfur can be used for many domestic purposes and especially for exporting, the molten sulfur needs to be converted to solid, generally particulate, sulfur. A number of different processes are known whereby molten sulfur is converted into solid particles. For example, in the "slate" process, molten sulfur is poured into large, thin layers, frequently in a continuous manner on conveyor belts. When cooled and solidified, the resulting thin "slates" of sulfur are mechanically crushed to produce particles in the desired size range. Equipment for sulfur slating processes is relatively expensive, although the sulfur produced in this manner is generally dry.
As disclosed, for example, in U.S. Pat. No. 4,389,356 to J. T. Higgins, sulfur prills (small pellets) are produced by discharging molten sulfur through small nozzle orifices located at the top of a prilling tower, the prills being formed as the sulfur droplets cool and solidify while falling through the air. The Higgins patent also discloses vibrating the sulfur discharge nozzles and the use of a counter-flow of cooling air. Sulfur prills may also be formed by cooling and solidifying the sulfur droplets from a prilling nozzle in water instead of air, as disclosed in U.S. Pat. No. 3,334,159 to Campbell. In general sulfur prills tend to be quite small in size and fairly light in weight.
U.S. Pat. No. 3,830,631 to D. C. Young et al. alternatively discloses the production of sulfur particles by spraying pressurized molten sulfur from a nozzle. The resulting molten sulfur stream is typically broken up into droplets by impinging water jets, which also cool and solidify the droplets. As a still further example, U.S. Pat. No. 4,595,350 to B. Harbolt et al. discloses the production of sulfur particles by discharging a thin sheet of molten sulfur downwardly into the path of water jets which disperse the sulfur into droplets which then solidify into particles.
These and other known processes produce, or may produce, particulate sulfur which is suitable for such purposes as agricultural use. However, for various reasons related to sulfur particle size, density and water content and the cost of the required processing equipment, these known processes either may not produce a sulfur product which is satisfactory for the substantial sulfur export market or may not be economical for relatively small sulfur producers to use for such market.
In this regard, the export market for particulate sulfur differs substantially from the domestic, largely agricultural, market for particulate sulfur. By way of explanation of this difference, sulfur, when exposed to air, is rapidly oxidized to sulfate ions (which is the dominant form absorbed by higher plants) by various sulfur-feeding microorganisms, such as chemoautotropic bacteria of the genus Thiobacillis, heterotropic bacteria, fungi and actinomycetes. Consequently, small, light weight sulfur particles, which collectively have a large surface area per unit weight and volume, are favored for direct agricultural use because such particles provide a large surface area upon which such microorganisms can feed.
A side effect is that the sulfate ions produced by sulfur-feeding organisms in time react with the water usually present in water-produced particulate sulfur to form sulfuric acid. Such formation of sulfuric acid is usually not a serious problem for particulate sulfur produced for domestic agricultural use because the sulfur is generally not stockpiled long enough for significant amounts of sulfuric acid to form. Moreover, any sulfuric acid which does form is not detrimental for agricultural purposes. Nevertheless, even for domestic, agricultural use, some water removal to limit the amount of sulfuric acid formed during storage may be desirable if the product is excessively wet as produced.
For the export market, however, small and/or porous, wet sulfur particles, which tend to produce sulfuric acid, are very undesirable. Often particulate sulfur is produced from a tank truck load of molten sulfur and, at least for small producers, the accumulation of ship load quantities of particulate sulfur may take several weeks or months. If the product being stockpiled has a large surface-to-volume ratio and is even fairly wet, appreciable amounts of sulfuric acid may be formed during storage. When the sulfur particles are stockpiled on concrete floors (as is most often the case), the sulfuric acid so formed attacks and damages the concrete. As a result, when the sulfur is later scooped off the concrete for shipping, some of the dissolved concrete material is unavoidably scooped up with the sulfur and causes undesirable product contamination. Furthermore, during shipboard transit to overseas destinations, any sulfuric acid present will cause corrosion of metal storage areas. Shipping surcharges which are applied to cover such corrosion damage to ships obviously affect the selling price and even the marketability of the product.
Apart from the sulfuric acid problem, particulate sulfur having a high water content is undesirable for export because buyers neither want to buy the water at the price paid for the sulfur nor pay the further cost of shipping the water. As a result, a surcharge is also generally applied to sulfur having a water content of more than about 0.5 percent (typical export standards require no more than about 2.0 percent water content). Since water content of sulfur particles is largely attributable to water adhering to particle surfaces, it is desirable for this reason alone that the total combined surface area of the particles for a given product weight or volume be made as small as is economically practical.
To otherwise reduce shipping costs which in turn affect selling price, the particulate sulfur should have a relatively high bulk density to thereby minimize the shipping volume. Moreover, more dense sulfur particles tend to be relatively non-porous and therefore generally enable a dryer product which, as above-mentioned, favorably affects shipping costs. Still further, dense sulfur particles generally provide less surface area for a given volume or weight and therefore, with the reduced water retention, typically result in a further reduction in the amount of sulfuric acid formed during stockpiling because of a lower sulfate ion production rate.
Thus, for at least the export market, particulate sulfur should have: (i) a relatively high bulk density, (ii) a relatively low surface area for a given weight or volume, and (iii) a relatively low water content. Moreover, it is apparent that any improved apparatus and processes for producing such an advantageous product should be economical to the extent that the produced sulfur can be competitively priced for the export market. The present invention is, therefore, principally directed to an economical apparatus and process for producing particulate sulfur having such desirable characteristics, not only for the export market but for other markets in which some or all of these characteristics may be advantageous or desirable.